You need to do a check to identify where the problem is:
On your starter solenoid there should be two big terminals and one little one. This small one has the wire that comes from the ignition switch and should energise when you turn said switch to the starting position. With a multimeter (or if unavailable 12v bulb) check that there are 12v relative to earth at this terminal when your assistant tries to start the engine.
If there are 12v then the problem is with the solenoid or its earthing and if not its the ignition switch or associated wiring.